A Google search will return hundreds of results. Preferably, choose 64x64 pixels as the image size. Then just save your favorite icon to '~/.local/share/icons' and edit the "Icon=google-plus.png" section from the code above to whatever the name of your favorite icon is.

Now, open your home directory and press Ctrl + H to see the hidden files. Navigate to '.local/share/applications' and drag and drop the newly created file 'google-plus.desktop' into the Unity Launcher.